Equipment help: Is the port length messing with my pictures?

[danilocesar]

Hey divers,

I have a pretty simple question which the more savvy shooters might know the answer pretty well, but it's annoying me for a while.
I do have a pretty old equipment:
* T1i camera
* Tokina 10-17 lens
* Ikelite housing
* 8'' glass dome (default from ikelite)

Ikelite recommends for that lens a "Lens Extension 2.75-Inch" [1] extension module. I do have/use a longer one, a 4.25 inch extension module.
My question is: Despite the border of the dome appearing a little bit when the zoom is maxed out (as a black shadow), is there any image-quality problem on using it that way?
Any change of losing sharpness or having distortions because of my setup?

Thanks,

[1] - Ikelite | DSLR Lens Port Recommendations

 

[bvanant]

In addition to some vignetting, you will lose corner sharpness (which is highly overrated in my opinion). You can either add an extension (thus moving the virtual image further from the lens) or add a +2 diopter, or if you are happy with your pics, just ignore it. The 4.25 inch extension will not be optimal in my experience.
Bill